1. Field of the Invention
The invention relates to a plasma display panel, and more particularly to a plasma display panel and a method of driving the same both of which are capable of stably displaying images even when much images are to be displayed.
2. Description of the Related Art
A conventional plasma display panel, a conventional method of driving the same and a conventional method of controlling a luminance in a plasma display panel are explained hereinbelow with reference to FIGS. 1 to 3.
FIG. 1 is a perspective broken view of a conventional plasma display panel suggested in Japanese Patent Application Publications Nos. 2000-11899 and 2001-76625, for instance.
A plasma display panel includes an electrically insulating front substrate 1A and an electrically insulating rear substrate 1B both of which are composed of glass.
On the electrically insulating front substrate 1A are formed transparent scanning electrodes 2 and transparent sustaining electrodes 3, and trace electrodes 4 composed of metal are formed on the scanning and sustaining electrodes 2 and 3 in order to reduce a resistance of the scanning and sustaining electrodes 2 and 3.
A first dielectric layer 9 is formed on the electrically insulating front substrate 1A such that the scanning and sustaining electrodes 2 and 3 are entirely covered with the first dielectric layer 9. On the dielectric layer 9 is formed a protection layer 10 for protecting the dielectric layer 9 from discharges. The protection layer 10 is composed of magnesium oxide, for instance.
On the electrically insulating rear substrate 1B are formed data electrodes 5 extending perpendicularly to the scanning and sustaining electrodes 2 and 3. A second dielectric layer 11 is formed on the electrically insulating rear substrate 1B such that the data electrodes 5 are entirely covered with the second dielectric layer 11.
On the second dielectric layer 11 are formed partition walls 12 extending in parallel with the data electrodes 5 and defining display cells (see FIG. 2) as units for displaying images.
Sidewalls of the partition walls and an exposed surface of the second dielectric layer 11 are covered with a phosphor layer 8 which converts ultra-violet rays generated by discharge in discharge gas, into visible light.
Spaces 6 sandwiched between the electrically insulating front and rear substrates 1A and 1B and partitioned by the partition walls 7 define discharge spaces 6 filled with helium (He), neon (Ne) or xenon (Xe) alone or in combination.
In the plasma display panel having the above-mentioned structure, surface discharge 100 is generated between the scanning electrodes 2 and the sustaining electrodes 3.
FIG. 2 is a plan view of the plasma display panel illustrated in FIG. 1, as viewed from a viewer.
A scanning electrode 2 and two sustaining electrodes 3 located adjacent thereto form two gaps therebetween, one of which is a primary discharge gap MG in which discharge is generated, and the other of which is a non-discharge gap SG in which discharge is not generated. Thus, a unit display cell 12 is defined by the partition walls 7 and the non-discharge gap SG.
The non-discharge gap SG is designed relatively long in order to reduce interference in discharges generated in display cells adjacent to each other in a direction in which the partition walls 7 extend. The non-discharge gap SG is generally designed four or five times longer than the primary discharge gap MG.
In order to reduce interference in discharges generated in display cells adjacent to each other in a direction in which the partition walls 7 extend, the partition walls 7 may be formed in the non-discharge gap SG.
Hereinbelow is explained a display operation of a display cell.
FIG. 3 is a timing chart showing waveforms of voltage pulses to be applied to electrodes in a conventional method of driving a plasma display panel.
As illustrated in FIG. 3, a fundamental cycle for driving the plasma display panel includes a preliminary discharge period (A) in which display cells are reset for causing discharges to be readily generated in the subsequent period (B), a scanning period (B) in which it is selected which display cell or cells is(are) to be turned on or off, a sustaining period (C) in which discharges are generated in all of the selected display cells, and a sustaining-elimination period (D) in which the discharges having been generated in the sustaining period (C) are terminated. Such a fundamental cycle is called a sub-field.
In the conventional method of driving a plasma display panel, reference voltages of surface electrodes comprised of the scanning and sustaining electrodes 2 and 3 are set equal to a sustaining voltage Vos to keep discharges generated in the sustaining period (C). Accordingly, with respect to the scanning and sustaining electrodes 2 and 3, a voltage higher than the sustaining voltage Vos is a positive voltage, and a voltage lower than the sustaining voltage Vos is a negative voltage. With respect to the data electrodes 5, a reference voltage is set equal to zero (0) volt.
In the preliminary discharge period (A), a positive serrate preliminary discharge pulse Pops is applied to the scanning electrodes 2, and concurrently, a negative rectangular preliminary discharge pulse Popc is applied to the sustaining electrodes 3.
The preliminary discharge pulse Pops is designed to have a wave-height greater than a threshold voltage at which discharge starts being generated between the scanning and sustaining electrodes 2 and 3. Hence, weak discharge is generated between the scanning and sustaining electrodes 2 and 3 when the preliminary discharge pulses Pops and Popc are applied to the scanning and sustaining electrodes 2 and 3, and, a voltage of the serrate preliminary discharge pulse Pops raises, thereby a voltage between the scanning and sustaining electrodes 2 and 3 exceeds the above-mentioned threshold voltage. As a result, negative wall charges are accumulated above the scanning electrodes 2, and positive wall charges are accumulated above the sustaining electrodes 3.
Following the preliminary discharge pulse Pops, a negative serrate preliminary discharge-eliminating pulse Pope is applied to the scanning electrodes 2. The sustaining electrodes 3 are kept at the sustaining voltage Vos.
By applying the negative serrate preliminary discharge-eliminating pulse Pope to the scanning electrodes 2, wall charges having been accumulated above the scanning and sustaining electrodes 2 and 3 are eliminated.
Herein, the term “eliminate” should not be limited to elimination of all of wall charges, but should be interpreted as including reduction in wall charges for smoothly generating discharges in the scanning period (B) and the sustaining period (C).
In the scanning period (B), all of the scanning electrodes 2 are kept at a base voltage Vobw, and then, a negative scanning pulse Pow is applied to the scanning electrodes 2 one by one, and concurrently, a data pulse Pod is applied to the data electrodes 5 in accordance with data to be displayed. The sustaining electrode 3 is kept at a positive voltage Vosw.
Ultimate voltages of the scanning pulse Pow and the data pulse Pod are determined such that a voltage across the scanning and data electrodes 2 and 5 does not exceed a threshold voltage at which discharge is generated between the scanning and data electrodes 2 and 5, if only one of the scanning pulse Pow and the data pulse Pod is applied to the scanning or data electrodes 2 or 5, but exceeds the threshold voltage, if both of the scanning pulse Pow and the data pulse Pod are applied to the scanning and data electrodes 2 and 5.
The voltage Vosw at which the sustaining electrodes 3 are kept in the scanning period (B) is determined such that a voltage across the scanning and sustaining electrodes 2 and 3 does not exceed a threshold voltage at which discharge is generated between the scanning and sustaining electrodes 2 and 3, even if the voltage Vosw is added to the scanning pulse Pow.
Accordingly, cross-discharge is generated between the scanning and data electrodes 2 and 5 only in a display cell in which the scanning pulse Pow is applied to the scanning electrodes 2 and the data pulse Pod is applied to the data electrodes 5.
When cross-discharge is generated between the scanning and data electrodes 2 and 5, since a voltage caused by the scanning pulse Pow and the voltage Vosw is applied across the scanning and sustaining electrodes 2 and 3, there is generated discharge also between the scanning and sustaining electrodes 2 and 3 with the cross-discharge acting as a trigger. The thus generated discharge is data-writing discharge.
As a result, positive wall charges are accumulated above the scanning electrode 2, and negative wall charges are accumulated above the sustaining electrodes 3 in a selected display cell.
Then, all of the scanning electrodes 2 are kept at the sustaining voltage Vos, and a first sustaining pulse Posf is applied to the sustaining electrode 3 in the sustaining period (C).
The sustaining voltage Vos is determined to be such a voltage that if a voltage caused by wall charges accumulated above the surface electrodes by data-writing discharge in the scanning period (B) is added to the sustaining voltage Vos, discharge will be generated, and if not, a voltage across the surface electrodes will not exceed a threshold voltage, and hence, discharge is generated between the surface electrodes.
Accordingly, sustaining voltage is generated only in a display cell in which there has been generated data-writing discharge in the scanning period (B), and hence, wall charges have been accumulated on above the surface electrodes.
Then, sustaining pulses Pos having a wave-height equal to the sustaining voltage Vos and having phases inverted to each other are applied to the scanning and sustaining electrodes 2 and 3. As a result, there is generated sustaining voltage only in a display cell in which discharge has been generated by the first sustaining pulse Posf.
In the subsequent sustaining period (D), the sustaining electrodes 3 are kept at the sustaining voltage Vos, and a negative serrate sustaining-elimination pulse Poe is applied to the scanning electrodes 2. As a result, wall charges having been accumulated above the surface electrodes are eliminated, and hence, the plasma display panel is returned back to its initial condition, that is, a condition observed prior to the application of the preliminary discharge pulses Pops and Popc to the scanning and sustaining electrodes 2 and 3 in the preliminary discharge period (A).
Herein, the term “eliminate” should not be limited to elimination of all of wall charges, but should be interpreted as including reduction in wall charges for smoothly generating discharges in the subsequent periods.
In the above-mentioned method, the scanning period (B) and the sustaining period (C) are temporally separated from each other. In some methods of driving a plasma display panel, steps to be carried out in the scanning and sustaining periods are carried out in temporally mixed condition. However, it is common in each of display cells that a preliminary discharge period, a scanning period and a sustaining period are carried out in this order.
Hereinbelow is explained a conventional method of controlling a luminance in a plasma display panel.
In a plasma display panel, images are displayed at gray scales in accordance with a sub-field process. This is because it is difficult to control a luminance of light-emission by modulating a voltage, and hence, it is necessary to vary a number of light-emission for controlling a luminance in a conventional AC type plasma display panel.
Herein, a sub-field process is a process in which a picture to be displayed with gray scales is divided into a plurality of binary images, and those binary images are successively displayed at a high speed to thereby reproduce the picture with gray scales by virtue of visual storage effect.
A picture is displayed generally in 1/60 seconds, and this is called one field. When images are displayed at 8 bit and 256 gray scales, one field is divided into eight sub-fields (SFs), and a luminance ratio 1: 2: 4: 8: 16: 32: 64: 128 is assigned to the sub-fields. Thus, by selecting a sub-field(s) in which light-emission is carried out in a selected display cell(s), in accordance with a luminance level of input signal, it would be possible to display images at a plurality of gray scales.
Each of the sub-fields is comprised of four periods, that is, the preliminary discharge period (A), the scanning period (B), the sustaining period (C) and the sustaining-elimination period (D). A luminance in each of the sub-fields can be controlled by varying a number of sustaining cycles in the sustaining period (C).
A number of sub-fields may be set greater than a number of bits in a gray scale to provide redundancy. This is advantageous for suppressing moving picture pseudo-frame, which is one of defectiveness unique to a plasma display panel.
A plasma display panel is required to have high accuracy for enhancing display quality.
In the above-mentioned conventional method of driving a plasma display panel, if a number of display lines is increased by accomplishing high accuracy, it is unavoidable that the scanning period (B) is rendered longer, and accordingly, the sustaining period (C) is rendered shorter.
For instance, it is assumed that a scanning pulse has a pulse width of 2 microseconds.
If VGA having 480 display lines is displayed in eight sub-fields, the scanning period (B) would be 7.68 milliseconds (2 μs×480×8=7.68 ms). Thus, the scanning period (B) occupies about 46% of one field.
If XGA having 768 display lines is displayed in eight sub-fields, the scanning period (B) would be 7.68 milliseconds (2 μs×768×8=12.288 ms). Thus, the scanning period (B) occupies about 74% of one field, which is equal to about a half of the same in VGA.
The reduction of the sustaining period (C) in duration causes a problem that a display luminance is reduced.
Furthermore, if a number of sub-fields is increased for suppressing moving picture pseudo-frame, there is caused a problem that the scanning period (B) is rendered longer, and hence, the sustaining period (C) is rendered shorter accordingly.
In order to avoid the scanning period (B) from being rendered longer when a number of display lines or a number of sub-fields is increased, for instance, a scanning pulse is designed to have a short width.
However, a short width of a scanning pulse causes a problem that a ratio at which data-writing discharge is generated is reduced, resulting in that images cannot be properly displayed.
Japanese Patent Application Publication No. 2000-123750 has suggested a plasma display panel including a front substrate and a rear substrate. A plurality of first electrodes is formed on the rear substrate, and a plurality of second and third electrodes are formed on the front substrate. At least one preliminary electrode is formed on the front substrate in parallel with the second and third electrodes.
Japanese Patent Application Publication No. 2002-100294 based on U.S. patent application Ser. No. 09/629,118 filed on Jul. 31, 2000 has suggested a plasma display panel including an upper glass substrate on which first and second sustaining electrodes are formed, and at least one preliminary electrode is further formed in parallel with the first and second sustaining electrodes. The preliminary electrode is adjacent to the first sustaining electrode.